cannot define constructor as protected or private! why?

--- A.php ----
require_once 'B.php';

class A
{
    public function __constructor($x){...}

    public function foo()
    {
        $b = B::get开发者_运维知识库Instance();
        ...
    }
}

--- B.php ----
require_once 'A.php';

class B extends A
{
    protected static $_instance = null;

    protected function __construct(){}

    public static function getInstance()
    {....}
}

PHP just stops interpreting the code as soon as it reaches line

protected function __construct(){}

and outputs everything before and nothing that would have been sent to the browser afterwards.

And as soon as I just take that line out, by changing it to

// protected function __construct(){}

everything works fine!?

I don't get that.

Any ideas?

I've just created a simple test-file to confirm whether this happens on my machine too, and I think I've found the answer. Take the following code:

<?php
error_reporting( E_ALL | E_STRICT );
class Foo {
    public function __construct( ) {
    }
}

class Bar extends Foo {
    protected function __construct( ) {
    }
}

When trying to execute that code, I get a fatal error: "PHP Fatal error: Access level to Bar::__construct() must be public (as in class Foo) in /home/berry/foo.php on line 12." That means you can't change the access level in a child class, if the parent has already defined the access level, which actually makes a lot of sense: PHP wouldn't know which constructor to call, I guess.

As a side note: by looking at your code, B extends A, and A uses B. Why exactly is that so, it seems like a strange construction to me? I'm guessing you actually want is composition, not inheritance.

You can define a constructor as protected or private. This code compiles runs just fine since OOP was rewritten for PHP/5:

<?php

class A{
    public function __construct(){
        echo 'Instance of A created' . PHP_EOL;
    }
}

class B{
    protected function __construct(){
        echo 'Instance of B created' . PHP_EOL;
    }
}

class C{
    private function __construct(){
        echo 'Instance of C created' . PHP_EOL;
    }
}

Of course, private constructors prevent you from creating an instance with the new keyword, but PHP will trigger a fatal error (it won't just stop running):

<?php

new A; // OK
new B; // Fatal error: Call to protected B::__construct() from invalid context
new C; // Fatal error: Call to private C::__construct() from invalid context

You can create custom static initiators:

<?php

class FancyClass {
    public static function init() {
        new self();
    }

    protected function __construct() {
        echo 'Instance created!';
    }
}

# This will not work.
new FancyClass();

# But this will work.
FancyClass::init();